This invention relates to a structure of a registration mark for electron beam exposure and more particularly, to a semiconductor device such as a gallium arsenide device or the like having a registration mark for electron beam exposure.
When an electron beam is radiated to a registration mark and a neighborhood thereof which is provided on an object such as GaAs semiconductor substrate having high resistivity in order to make registration, build-up of charge occurs on this registration mark, and therefore, the mark position can not be accurately detected by the stored charge in the mark.
In electron beam exposure techniques for forming microfine pattern in the fabrication of semiconductor devices or the like, the amount of the dosage of electron beam into a resist for electron beam exposure is determined by the resist sensitivity. In detecting the registration mark, on the other hand, detection is carried out by detecting scattered electrons with respect to the incident electron beam upon the registration mark. In other words, the change in the scattered electrons (reflected electrons, secondary electrons), that occurs when the electron beam is scanned from the flat portion to step portion of the registration mark, is detected by a scintillator detector or the like so as to confirm the position of the registration mark. In this case, scanning of the electron beam on the registration mark is carried out repeatedly in order to obtain high detection accuracy, that is, to detect the mark position with a high level of accuracy, and to obtain a high S/N ratio. Therefore, the amount of dosage of the electron beam to be radiated to the mark becomes higher level than that of the electron beam to be applied to the resist to form the device regions.
When the registration mark is disposed on a semiconductor substrate having high resistivity such as GaAs, therefore, the charge is built up at this mark and the scanning direction of the incident electrons is modulated by the charged electrons in the mark so that the detection position accuracy of the registration mark is adversely affected.
The operation for detecting the registration mark is essentially effected as a pre-operation for forming an etching pattern, and therefore, the position of the registration mark must be accurately detected. Namely, above mentioned charged electrons in the registration mark on the substrate having high resistivity must be effectively rejected so that the incident electron beam is not modulated by the charged electrons. For rejecting and preventing the charged electrons, in the prior art, the impurity region of high impurity concentration is formed in the substrate under the registration mark by ion injection techniques. However, thermal annealing treatment is necessary for forming the impurity region after ion injection and such a thermal treatment changes sensitively the characteristics of the device region. Moreover, such a impurity region cannot disperse the charged electrons effectively. Furthermore, the techniques can not be applied if the substrate consists of a dielectric other than the semiconductor, such as glass, for example.